The need for further miniaturization of microcircuitry places increased demands on the photolithographic lenses that are used for projection printing the microcircuitry. Since such lenses are large and complex and contain many optical elements, they present a significant manufacturing challenge to the achievement of finer resolution into the submicron range. Our invention contributes to finer resolution by addressing ways of reducing the distortion involved in mounting optical elements. Although our mounting improvements were made with photolithographic projection lenses in mind, they may also be usable in other optical systems requiring exceptional accuracy in the mounting and assembly of optical elements.
Photolithographic projection lenses have previously been assembled in a traditional way by arranging lens elements and lens support elements within a large lens barrel. Although the barrel can hold a number of elements in place, its presence prevents any direct observation of the exact location of the cell subassemblies within the barrel. It is also especially difficult to go back into the assembled lens for fine adjustments, because many lens elements and their supports may have to be removed to adjust a lens element in the middle of the barrel. Any removed elements cannot be returned exactly to the positions they previously occupied, so that every adjustment of the lens assembly creates accuracy problems.
We have found that these lens barrel difficulties can be avoided by mounting lens elements within individual annuli that are assembled in a stack, without using a lens barrel. This makes the position of each individual element determinable from the position of its annulus; and it becomes somewhat simpler to disassemble the stack by disconnecting one annulus from another, allowing one lens element to be adjusted with minimal disruption of other lens elements. The lens supporting annuli can be accurately machined and carefully interconnected to provide a stable support for the lens elements they contain, and such an assembly of stacked annuli offers possibilities for improving the accuracy of the mountings of the assembled elements.
The stacked annuli approach suffers from a problem not generally experienced in lens barrel assemblies, and this involves inaccuracies introduced by interconnecting the annuli in the stack. We have investigated and have devised a solution for this problem. Our mount allows us to take advantage of the benefits of the stacked annuli approach, while also mounting optical elements to a high degree of accuracy that reduces the degradation of lens imagery and thereby enables more accurate printing of smaller circuit elements.